Facial expressions are processed using different mechanisms originating in brain regions specific to particular emotions. This study demonstrates perceptual differences between emotions that may relate to physiological differences in processing. Changes to the height and shape of eyebrows, amount of visible sclera, upper and lower eyelid positions, width of nose and curvature and size of mouth were applied to synthetic faces based on Ekman's work to produce expressions of happiness, sadness, fear and anger. Two alternative forced choice experiments were used to determine the percent change from a neutral face required to reach threshold (75% correct). In a second experiment, 4 alternative forced choice was used to determine the amount of physical change to distinguish among all four individual emotions. Results suggest that fearful expressions require much less physical change than happy, angry and sad facial expressions to distinguish between emotion and neutral. Fear is harder to identify from among emotions, possibly because it is frequently mistaken for sadness. Fear and anger are rarely confused. The highly sensitive recognition mechanisms for fearful expressions may be related to automatic physiological changes that occur when viewing fearful expressions (Whalen et al., 1998 The Journal of Neuroscience 18, 411–418). Another finding suggests that happiness is much easier to distinguish from other emotions than from neutral. These findings support the hypothesis that individual emotions are processed in specific ways. Additional experiments quantify reductions in emotion discrimination in peripheral vision.